DFT approach towards accurate prediction of ¹H/¹³C NMR chemical shifts for dipterocarpol oxime

A computational NMR approach for accurate predicting the ¹H/¹³C chemical shifts of triterpenoid oximes featuring the screening of 132 DFT methods was demonstrated. Efficiently synthesized dipterocarpol oxime was employed as a model compound. Six highest accurate methods from the screening generated the root-mean-square-error (RMSE) values in the range of 0.84 ppm (0.55 %) to 1.14 ppm (0.75 %) for calculated ¹³C shifts. For ¹H results, simple, economical 6-31G basis set unexpectedly outperformed other more expensive basic sets; and the couple of it with selected functionals provided high accuracy shifts (0.0617 ppm (1.49 %) ≤ RMSE ≤ 0.0870 ppm (2.04 %)). These computational results strongly supported the proton and carbon assignments of the oxime including the difficult ones of diastereotopic methyl groups, the methyl groups attached to an internal olefin, and diastereotopic a-protons.